The earliest hook accessible from external code is muplugins_loaded. In order to use it, create a directory mu-plugins in your wp-content directory and put a PHP file into that directory.
Sample code, will have side effects(!):
add_action( 'muplugins_loaded', function() {
print current_filter();
});
This is, of course, not "before anything else in ...

plugins_loaded is going to load this form much too early, even if you weren't getting errors. The form is loading before the opening <html tags. You need to select better hooks for this-- admin_notices maybe, but it is hard to tell exactly what you want.
Second, you need to separate your handler:
function process_wpse_194468() {
if ...

I wrote this plugin to do what you're asking - Simply Show Hooks. It shows you where all the action and filter hooks are, inline, in order, on the page you're looking at, and all the functions that have been hooked to them. Hope this helps :)